Solid state lasers (SSL) are well known. Examples of solid state lasers include high average power (HAP) solid state lasers and high-energy lasers (HEL). Such lasers may be used for a variety of military and commercial applications. Military applications include target illumination/designation for air defense and precision strike. Commercial applications include cutting and welding in the aerospace, automotive and shipbuilding industries.
In such solid state lasers, gain uniformity and low optical phase error are critical with respect to achieving good beam quality, as well as with respect to facilitating operation at high average power. By maintaining good beam quality, more powerful and more economical high average power solid state lasers may be constructed.
However, contemporary high average power solid state lasers suffer from deficiencies that tend to detract from desirable beam quality. For example, the beams of contemporary high average power solid state lasers may pass through thermal eddy currents that perturb the optical phase fronts thereof. Thus, such thermal eddy currents cause undesirable degradation to beam quality.
More particularly, solid state lasers use one or more solid state gain elements that are doped with rare earth ions. During laser operation these ions are excited by optical radiation to a laser transition. A portion of the excitation energy is unavoidably converted into heat which is dissipated within the solid state gain element. To sustain continuous operation of the laser, the solid state gain element is cooled.
For practical reasons, solid state laser coolant is typically delivered at about room temperature. For efficient heat transfer, the solid state gain element generally operates at a temperature that is significantly higher than the coolant temperature.
In contrast, other solid state laser components such as lenses, mirrors, an optical bench and enclosures typically are un-cooled and operate at approximately ambient (room) temperature. A laser beam is transported by such lenses and mirrors into and out of the solid state gain element.
The space between the solid state gain element and the beam transport components is often very large compared to the size of the gain elements. This space is typically filled with a gas such as air, nitrogen, or helium. Since the gas is in intimate contact with various solid state laser components at ambient temperature, the gas is also generally at approximately ambient temperature.
During laser operation, the temperature of the gain elements typically rises well above ambient temperature. This is especially true with respect to the gain element's surfaces through which the laser beam enters or exits the gain element, because such surfaces are typically not cooled. The surfaces of the gain element heat the gas in the vicinity of these surfaces. This heating of the gas generates thermal gradients and thermal eddy currents.
A laser beam passing through such thermal eddy currents experiences both temporal and spatial variations in index of refraction. As a result, the optical phase fronts of the beam are undesirably perturbed. This perturbation translates into a reduction in beam quality.
The reduction in beam quality may be small enough to be neglected in many low or medium average power solid state laser systems. However, the reduction in beam quality is very detrimental to the performance of high average power solid state laser systems, such as those used for military applications and for precision material processing, e.g., cutting and welding.
Thus, it is desirable to provide a way to mitigate the detrimental effects of thermal eddy currents in the gas atmosphere of a high average power solid state laser system. This is especially true for those thermal currents that pass through the laser beam.